Always have two overloaded versions of functions that take
char * and const char * parameters. Declare (but don't define if not needed)
a function that takes const char* as a parameter when you have defined
a function that accepts a non-const char* as a parameter.
#include <iostream>
#include <iomanip>
static void foo (char *s) {
std::cout << "non-const " << std::hex << static_cast <void *>(s) << std::endl;
}
static void foo (char const *s) {
std::cout << "const " << std::hex << static_cast <void const *>(s) << std::endl;
}
int main (void)
{
char * c1 = "Literal String 1";
char const * c2 = "Literal String 1";
foo (c1);
foo (c2);
foo ("Literal String 1");
//*c1 = 'l'; // This will cause a seg-fault on Linux.
std::cout << c2 << std::endl;
return 0;
}
Because of default conversion rule from string literal to char *,
the call to foo using in-place literal goes completely undetected
through the eyes of compiler's type system.
Interestingly enough, the addresses of all the identical string literals
is the same, irrespective of whether it is assigned to const or non-const.
Internally though, they are stored on the const DATA page and modifying
them causes a seg-fault.
char * and const char * parameters. Declare (but don't define if not needed)
a function that takes const char* as a parameter when you have defined
a function that accepts a non-const char* as a parameter.
#include <iostream>
#include <iomanip>
static void foo (char *s) {
std::cout << "non-const " << std::hex << static_cast <void *>(s) << std::endl;
}
static void foo (char const *s) {
std::cout << "const " << std::hex << static_cast <void const *>(s) << std::endl;
}
int main (void)
{
char * c1 = "Literal String 1";
char const * c2 = "Literal String 1";
foo (c1);
foo (c2);
foo ("Literal String 1");
//*c1 = 'l'; // This will cause a seg-fault on Linux.
std::cout << c2 << std::endl;
return 0;
}
Because of default conversion rule from string literal to char *,
the call to foo using in-place literal goes completely undetected
through the eyes of compiler's type system.
Interestingly enough, the addresses of all the identical string literals
is the same, irrespective of whether it is assigned to const or non-const.
Internally though, they are stored on the const DATA page and modifying
them causes a seg-fault.
Comments
Could you explain why the *c1 = 'l' segfaults under Linux? To me it looks like modifying a proper (non const) string?
Thanks!
The rational behind such a decision can be attributed to a number of reasons. One example is, as you've said, the literals being in the DATA segment -- modifying the same is tantamount to creating a self-modifying program.
When you declare a function taking a non-const char pointer, you are also declaring your intention to potentially change the input parameter. If you really are sure that you would never change the input parameter string use const char pointer instead. Now assuming that you do intend to change the input parameter string, you declared it as a non-const char pointer and someone out there passed in a string literal to it. That is a problem. Your function will try to change the string literal which is undefined by standard. To avoid this from happening you can take some help from the compiler. If you just declare a function of the same name and rest of the signature, taking const char pointer as input parameter, compiler will pick that up when someone out there uses a string literal to call it. const char pointer is a better match than non-const char pointer for a string literal. But because it is undefined (remember you just declared it) program won't link.
test.cpp|16 col 17| warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
so this advice is a bit obsolete